In the Kraft process for converting wood chips into cellulose the wood chips are cooked under pressure in an aqueous alkaline solution containing Na.sub.2 S whereby the cellulose is chemically separated from the other constituents of the wood chips. The spent cooking liquid is called black liquor and must be disposed of in such a way that its chemical values are recovered, usually in a recovery furnace such as a waste heat boiler (see Kirk-Othmer, Vol. 11, pages 575 and 576).
The black liquor is sprayed into the recovery furnace where water in the black liquor evaporates and the black liquor is immediately "dried" as the drops descend toward a hearth. The "dried" drops are at least partially burned and the unburned residue ultimately forms a bed of char (carbon) and a smelt, containing chemicals potentially useful in the pulping process. Any Na.sub.2 SO.sub.4 in the smelt is reduced to Na.sub.2 S when it contacts the char thereby regenerating a chemical required in the Kraft process cooking liquor.
At many Kraft mills, the amount of black liquor which can be fired into a recovery boiler is limited by the capacity of the recovery boiler, i.e. by the maximum amount of steam that can be produced safely. The amount of black liquor which can be fired into a recovery boiler to produce heat which is recovered as steam is also limited by the rate of deposit formation on the upper pendant heat transfer surfaces. The rate of deposit formation is determined by the impaction rate of droplets on the upper pendant heat transfer surfaces and by the physical state of the droplets. The impaction rate is a function of droplet size and the upward velocity of gases in the main furnace space. The probability of a particle attaching itself to a heat transfer surface depends on its state. When a droplet is semi-molten (i.e. tacky) it tends to condense upon contact with the cooler heat transfer surface. If the temperature of the particle entering the upper heat transfer surface is at or above that required for a semi-molten state, deposition will occur. When additional black liquor is injected into the boiler, additional air must be added, and the amount of energy released is increased. The rate of deposit formation will be increased because of the extra upward gas momentum and may increase if the temperature of the particles impacting the heat transfer surfaces rises above the tacky temperature. The maximum firing rate (i.e. capacity) of the boiler is reached when the rate of deposition is equal to the rate at which the deposit can be removed, by steam cleaning or by other means.